The present invention relates to a video signal generating device in which a solid-state color image sensor detects the optical image of an object to provide electrical video signals utilized to form a luminance signal and color difference signals.
In general, a video signal generating device provided in an electronic still camera or color video camera employs a solid-state image sensor having color filters for producing primary or complementary color signals so as to produce luminance signals and color difference signals according to the color video signals outputted by the solid-state image sensor.
FIG. 1 shows the arrangement of a conventional video signal generating device in a double-sensor type color camera or the like. As shown in FIG. 1, a two-color separation dichroic prism 2 is provided behind an image pickup lens 1, the prism 2 being used to separate the incident light beam into green and red-blue beams. An optical image formed by the red and blue light beams is received through an optical low-pass crystal filter 16 on a red-and-blue image sensor 3, on the surface of which is formed a red (R) and blue (B) stripe color filter array, the picture elements of which are horizontally scanned to provide red (R) and blue (B) color video signals. On the other hand, an optical image formed by the green light beam is received through an optical low-pass crystal filter 17 on a green image sensor 4, on the surface of which a green (G) color filter is provided, the picture elements of which are horizontally scanned to provide a green (G) color video signal.
The red, green and blue color video signals are outputted sequentially with a predetermined timing. The green color video signal G is applied through a preamplifier 5 to a low-pass filter 7 having a passband of from 0 to 4.2 MHz, where it is converted into a green signal G.sub.o whose frequency is limited by the passband. The output signal G.sub.o is applied to a subtractor 10 and through another low-pass filter 11. The low-pass filter 11, which has a passband of from 0 to 0.7 MHz, outputs a green signal G.sub.L whose frequency is limited by the passband. The output signal G.sub.L is applied to matrix circuit 14 (described below in detail) and to the subtraction input terminal (-) of the subtractor 10. On the other hand, the red color video signal R, sampled by a sample-and-hold circuit 8, is applied to a low-pass filter 12 having a passband of from 0 to 0.7 MHz, where it is converted into a red signal R.sub.L whose frequency is limited by this passband. Similarly, the blue color video signal B, sampled by a sample-and-hold circuit 9, is applied to a low-pass filter 13 having a passband of from 0 to 0.7 MHz, where it is converted into a blue signal B.sub.L Whose frequency is limited by the passband. The red signal R.sub.L and the blue signal B.sub.L are applied to the matrix circuit 14. In the subtractor 10, the green signal G.sub.o and the low frequency green signal G.sub.L are subjected to subtraction, as a result of which a high frequency luminance signal Y.sub.H (Y.sub.H =G.sub.o -G.sub.L) having a frequency band of from 0.7 to 4.2 MHz is applied to the matrix circuit 14.
In the matrix circuit 14, a low frequency luminance signal Y.sub.L having a frequency band of from 0 to 0.7 MHz is formed according to the red, blue and green signals R.sub.L, B.sub.L and G.sub.L having a frequency band of from 0 to 0.7 MHz, and the low frequency luminance signal Y.sub.L and the high frequency luminance signal Y.sub.H being subjected to addition to form the luminance signal. Further in the matrix circuit 14, color difference signals R.sub.L -Y.sub.L and B.sub.L -Y.sub.L are formed according to the red and blue signals R.sub.L and B.sub.L and the low frequency luminance signal Y.sub.L. These signals Y, R.sub.L -Y.sub.L, and B.sub.L -Y.sub.L are supplied to a color encoder circuit 15, which, in turn, outputs a video signal conforming to NTSC system requirements, for instance.
FIGS. 2A to 2F illustrate frequency characteristics of the signals produced in the above-described signal processing operations. As shown in FIGS. 2B, 2D and 2F, respectively, the red, blue and green signals R.sub.L, B.sub.L and G.sub.L, the low frequency luminance signal Y.sub.L and the color difference signal R.sub.L -Y.sub.L and B.sub.L -Y.sub.L have a frequency band of from 0 to 0.7 MHz. As shown in FIG. 2C, the high frequency luminance signal Y.sub.H has a frequency band of from 0.7 MHz to 4.2 MHz (with the shaded part being omitted). The luminance signal Y, being finally formed through the addition of the high frequency luminance signal Y.sub.H and the low frequency luminance signal Y.sub.L, has a frequency band of from 0 MHz to 4.2 MHz, as indicated in FIG. 2E.
A conventional video signal generating device in a single-sensor type color camera will be described with reference to FIG. 3. An optical low-pass crystal filter 41 is disposed behind an image pickup lens 21. A color filter 22 having a stripe-shaped or mosaic-shaped red (R), blue (B) and green (G) color filter array is attached to a solid-state image sensor 23. In the sensor 23 electrical signals produced from the picture elements are horizontally scanned. The output of the sensor 23 is supplied to a preamplifier 24, which outputs a color video signal. The color video signal thus outputted is supplied to a color separation circuit 26 to obtain the color signals R, G and B.
The green signal G is applied to a low-pass filter 27 having a passband of from 0 to 4.2 MHz, where it is converted into a green signal G.sub.o whose frequency is limited by the passband. The green signal G.sub.o is supplied directly to one input terminal (+) of a subtraction circuit 31, while it is supplied through another low-pass filter 30 having a passband of from 0 to 0.7 MHz to the other input terminal (-) of the subtraction circuit 31; That is, a green signal G.sub.L is applied to the minus input terminal (-) of the circuit 31. The green signal G.sub.L is further applied to a process circuit 32 where it is subjected to gamma correction, for instance. The output of the process circuit 32 is applied to one input terminal of a luminance signal amplifier circuit 35. On the other hand, the red signal R is supplied to a low-pass filter 28 having a passband of from 0 to 0.7 MHz, where it is converted into a red signal R.sub.L whose frequency is limited by the passband. The red signal R.sub.L, after being subjected, for instance, to gamma correction by a process circuit 33, is applied to another input terminal of the luminance signal amplifier circuit 35, and to a modulation circuit 36 for forming a color difference signal. Similarly, the blue signal B is supplied to a low-pass filter 29 having a passband of from 0 to 0.7 MHz, where it is converted to a blue signal B.sub.L whose frequency is limited by the passband. The blue signal B.sub.L, after being subjected, for instance, to gamma correction by a process circuit 34, is applied to the remaining input terminal of the luminance signal amplifier circuit 35 and to a modulation circuit 37 for forming a color difference signal.
The luminance signal amplifier circuit 35 forms a low frequency luminance signal Y.sub.L having a frequency band of from 0 to 0.7 MHz according to the red, blue and green signals, R.sub.L, B.sub.L and G.sub.L, the frequencies of which have been limited by the passband of 0 to 0.7 MHz of the low-pass filters 28, 29 and 30, respectively. The modulation circuit 36 forms a color difference signal R.sub.L -Y.sub.L according to the red signal R.sub.L and the low frequency luminance signal Y.sub.L. Similarly, the modulation circuit 37 forms a color difference signal B.sub.L -Y.sub.l according to the blue signal B.sub.L and the low frequency luminance signal Y.sub.L. Sub carriers S.sub.B1 and S.sub.B2 different in frequency from one another are supplied to the modulation circuits 36 and 37, respectively.
In the subtraction circuit 31, the two input signals G.sub.o and G.sub.L are subjected to subtraction, as a result of which a high frequency luminance signal G.sub.H (G.sub.H = G.sub.o -G.sub.L) having a frequency band of from 0.7 to 4.2 MHz is produced. In a mixer 38 the high frequency luminance signal G.sub.H and the low frequency luminance signal Y.sub.L are mixed to produce a luminance signal Y having a frequency band of from 0 to 4.2 MHz. Another mixer 39 mixes the color difference signals R.sub.L -Y.sub.L and B.sub.L -Y.sub.L to provide an output signal. A third mixer 40 mixes the luminance signal Y, the output signal of the mixer 39, and a synchronizing signal S.sub.c to output a video signal conforming to the NTSC system, for instance.
The above-described conventional video signal generating devices are however, disadvantageous in the following points:
In the double-sensor type color device shown in FIG. 1, the subtractor 10 forms the high frequency luminance signal Y.sub.H utilizing only the green signal produced by the green image sensor 4. Assuming that the optical image has substantially no green (G) color video signal, there is a possibility that it may be somewhat difficult to obtain the high frequency luminance signal Y.sub.H. As a result, luminance distortion which may be caused by the absence of a green component, is produced between the luminance signal Y.sub.H and the red and blue signals outputted by the red and blue image sensor 3. Furthermore, since the green and red and blue sensors are separately provided, color shift due to picture element shift is liable to occur because it is difficult to manufacture the device with a very high accuracy. In addition, because the image sensors 3 and 4 are covered by color filters, the device generally has a reduced light-detecting sensitivity.
In the single sensor type video signal generating device as shown in FIG. 3, employment of the stripe-type color filter 22 prevents the occurrence of color Moire; however, the resolution of this device is limited. If another color filter array is employed and the number of picture elements for generating the green signal is made larger than the number of picture elements for the red and blue signals, the resolution is improved, but color Moire will then occur.
As is apparent from the above description, in the conventional video signal generating device, improving the resolution is not compatible with preventing color Moire. There thus heretofore been available no technique for solving the two problems at the same time.